1. Field of the Invention
The present invention relates to a copier, printer facsimile apparatus or similar image forming apparatus. More particularly, the present invention relates to an image forming apparatus of the type transferring toner images sequentially formed on photoconductive drums or similar image carriers to an intermediate image transfer belt or similar first image transfer body one above the other and then transferring the resulting composite toner image to a recording medium or similar second image transfer body.
2. Description of the Background Art
To meet the increasing demand for color copies, an electrophotographic image forming apparatus is spreading for medium- and high-speed applications while an ink jet type image forming apparatus is predominant for low-speed applications. Particularly, a tandem color image forming apparatus is feasible for high-speed applications and includes a plurality of photoconductive drums or image carriers arranged side by side in the direction of sheet conveyance. Also feasible for high-speed applications is an image forming apparatus configured such that a toner image is transferred to a sheet or second transfer body by way of an intermediate image transfer belt or first transfer body.
Japanese patent Laid-Open Publication No. 10-246995, for example, discloses a tandem color image forming apparatus including four photoconductive drums arranged side by side in a direction in which a belt conveys a sheet. A light beam issuing from a particular optical writing unit scans each drum in the axial direction of the drum, i.e., the main scanning direction, forming a latent image on the drum. Developing units each being assigned to a particular drum develop such latent images with toners of different colors, i.e., cyan, magenta, yellow and black, thereby producing corresponding toner images. The toner images are sequentially transferred from the drums to a sheet being conveyed by the belt one above the other by chargers. After the resulting composite toner image has been fixed on the sheet, the sheet or print is driven out of the apparatus to a print tray. In this manner, a four-color or full-color image can be formed on a sheet only if the sheet is conveyed via the consecutive image transfer positions one time.
In another tandem color image forming apparatus, an intermediate image transfer belt is substituted for the belt stated above. In this type of apparatus, the toner images of four different colors are superposed on each other on the intermediate image transfer belt and then transferred to a sheet.
Problems to which the present invention addresses will be described hereinafter.
[Problem 1]
In the tandem color image forming apparatus of the type using the intermediate image transfer belt (simply belt hereinafter), toner images of different colors are sequentially transferred from the drums to the belt one above the other, forming a color image. Therefore, if the toner images are shifted from each other on the belt, then the colors of the color image are shifted from each other. Some different measures against such color shifts are taught in, e.g., Japanese Patent No. 2,929,671 and Japanese Patent Laid-Open Publication Nos. 63-11967 and 59-182139. Also, color shifts to occur when the drums and belt or sheet are moved at different speeds are discussed in, e.g., Kido and Iijima “Studies on Slip Transfer Mechanism”, Fuji Xerox Technical Report, No. 13 (Technical Report hereinafter).
In the tandem color image forming apparatus, even if the drums differ in eccentricity and radius from each other, the color images on the belt are free from color shifts only if the drums rotate at the same angular velocity and if the speed of the belt is constant. However, if gears included in a driveline assigned to the drums or the belt have eccentricity, then the angular velocities of the drums or the moving speed of the belt varies even though a motor or drive source may rotate at a constant speed, resulting in color shifts, as discussed in Technical Report and various publications.
In light of the above, Japanese Patent No. 2,929,671 mentioned earlier proposes to make an integral multiple of the period of variation ascribable to, e.g., the gears equal to a period of time necessary for each drum to rotate from an exposure position to an image transfer position. Also, Laid-Open Publication No. 63-11967 proposes to make an integral multiple of the period of variation of the drum driveline equal to a period of time necessary for the belt or the sheet to move between nearby drums. Further, Laid-Open Publication No. 59-182139 proposes to make an integral multiple of the period of rotation of a belt drive roller equal to a period of time necessary for the belt or the sheet to move between nearby drums.
We, however, found that none of the above conventional measures could obviate the expansion or the contraction of a pixel in the image transferred from the belt to the sheet and ascribable to the periodic speed variation of the belt. This is presumably because when the speed of the belt periodically varies, the belt speed varies between the primary transfer of a given pixel from the drum to the belt and the secondary transfer of the same pixel from the belt to the sheet, causing the pixel to expand or contract. Technical Report or the other publications do not address to the expansion and contraction of pixels ascribable to the periodic speed variation of the belt.
[Problem 2]
When a speed difference or relative speed between the drum and the belt, sheet or similar first image transfer body, as measured at the first image transfer position, increases, a pixel expands or contracts at the first image transfer position and lowers image quality, as will be described hereinafter.
Assume that a speed difference or slip occurs between the drum and the belt at the first image transfer position where they contact each other. Then, the line width of an image varies, i.e., expands or contracts by an amount δI:
 δI=(W1+Iw)·ΔV/Vd  Eq. (1)
where ΔV denotes a difference between the peripheral speed Vd of the drum and the peripheral speed Vb of the belt (Vd−Vb), and W1 denotes the width of a nip between the drum and the belt at the first image transfer position. The amount δI refers to a difference between the width Iw of a line image formed on the drum and the width of the corresponding line image formed on the belt.
The Eq. (1) indicates that as the speed difference ΔV (=Vd−Vb) increases, the amount of variation δI of the line width transferred from the drum to the belt increases. Further, the Eq. (1) indicates that the toner image formed on the drum is transferred to the belt while being rubbed, and that the amount δI varies due to the variation of the nip width W1. The nip width W1 varies in accordance with drum radius as well and generally increases with an increase in drum radius.
Assume that the angular velocity of the drum has a constant value of ωo, that the drum has a radius of Ro, and that the length of an exposed pixel for a unit time is Ie=Roωo. Then, when the drum has a radius of Ro+ΔRo, the length I of the exposed pixel is increased by Roωo for a unit time, as produced by:
 I=(Ro+ΔRo)ωo=Ie+ΔRoωo  Eq. (2)
Assuming that the belt speed Vb is Roωo, then a speed difference ΔV=ΔRoωo occurs between the drum surface and the belt at the first image transfer position. As a result, the pixel is contracted by the length δI derived from the Eq. (1), as produced by:                                                                         δ                ⁢                                                                   ⁢                I                            =                                                                    (                                                                  W                        1                                            +                      I                                        )                                    ·                  Δ                                ⁢                                                                   ⁢                                  V                  /                  Vd                                                                                                        =                                                                    (                                                                  W                        1                                            +                      Ie                      +                                              Δ                        ⁢                                                                                                   ⁢                        Ro                        ⁢                                                                                                   ⁢                        ω                        ⁢                                                                                                   ⁢                        o                                                              )                                    ·                  Δ                                ⁢                                                                   ⁢                                  Ro                  /                                      (                                          Ro                      +                                              Δ                        ⁢                                                                                                   ⁢                        Ro                                                              )                                                                                                          Eq        .                                   ⁢                  (          3          )                    
It follows that the expansion ΔRoωo of the pixel for a unit time at the time of exposure is contracted by the amount produced by the Eq. (3). Particularly, when the nip width W1 at the first image transfer position is zero, the pixel is contracted by ΔRoωo. More specifically, the discussion that when the angular velocity of the drum is constant, the pixel length remains the same even if the drum radius is irregular holds only when the nip width W1 is zero. This is also true when the drum has eccentricity.
If the influence of the nip width W1 is not negligible in the Eq. (3), then an error or contraction ofCe=W1·ΔRo/(Ro+ΔRo)occurs in the pixel length. More specifically, the pixel is expanded or contracted due to the nip width W1, as expressed as:                                                                         δ                ⁢                                                                   ⁢                I                            =                                                (                                                            W                      1                                        +                    Ie                    +                                          Δ                      ⁢                                                                                           ⁢                      Ro                      ⁢                                                                                           ⁢                      ω                      ⁢                                                                                           ⁢                      o                                                        )                                ⁢                                                                   ⁢                Δ                ⁢                                                                   ⁢                                  Ro                  /                                      (                                          Ro                      +                                              Δ                        ⁢                                                                                                   ⁢                        Ro                                                              )                                                                                                                          =                                                                                          W                      1                                        ·                    Δ                                    ⁢                                                                           ⁢                                      Ro                    /                                          (                                              Ro                        +                                                  Δ                          ⁢                                                                                                           ⁢                          Ro                                                                    )                                                                      +                                  Δ                  ⁢                                                                           ⁢                  Ro                  ⁢                                                                           ⁢                  ω                  ⁢                                                                           ⁢                  o                                                                                        Eq        .                                   ⁢                  (          4          )                    
When the speed variation between the drum and the belt or similar first image transfer body at the first image transfer position is reduced, the following advantage is achievable. For example, assume that the belt speed Vb is Roωo, and that the drum angular velocity is varied such that the moving speed at the first image transfer position becomes zero when the drum radius reaches Ro+ΔRo. Then, the drum angular velocity ω is derived from (Ro+ΔRo)ω=Vb=Roωo, as follows:ω={Ro/(Ro+ΔRo)}ωo  Eq. (5)
Therefore, the exposed pixel length Ie for a unit period of time is (Ro+ΔRo)ω=Roωo, meaning that the length Ie does not increase. Because the speed difference ΔV is zero at the image transfer position, there holds δI=(W1+Iw)·ΔV/Vd=0. In this case, an image free from expansion and contraction ascribable to the influence of the nip width W1 is achieved. More specifically, the smaller the speed difference ΔV at the first image transfer position, the less the influence of the nip width W1 on the image.
However, even if the speed difference ΔV is reduced at the design stage, any eccentricity of the drum or any variation of the belt speed ascribable to the eccentricity of the belt drive roller is likely to cause the speed difference ΔV to periodically increase. Should the speed variation ΔV increase, the pixels would be expanded or contracted at the first image transfer position due to the influence of the nip width W1. None of Technical Report and other publications even mentions the expansion or the contraction of pixels at the first image transfer position ascribable to the above cause.
Technical Report describes the following in relation to the degradation of image quality to occur in the image transferring step, i.e., degradation to occur at the nip for image transfer. According to Technical Report, a line width of 42.3 μm starts increasing little by little when the moving speed of the surface of an intermediate image transfer body (roller) exceeds about +0.5% of the moving speed of the surface of a drum (see Photo 1 and FIG. 9 of Technical Report). A specific procedure for calculating influence of the eccentricity of the drum and the irregularity of drum radius on the above surface moving speed will be described hereinafter. Assume that the drum radius is 30 mm and that irregularity in radius is ±30 μm, and that eccentricity is ±30 μm. The drum surface speed (peripheral speed) at the first image transfer position is assumed to be about ±0.3% when the drum is rotating at a constant angular velocity in terms of probability tolerance. It follows that if the description of Technical Report is true, then it is likely that the line width periodically increases in synchronism with the variation of the drum speed. Further, it is likely that the variation of the speed difference at the first image transfer position increases due to other factors: including the speed variation of the belt, which is the intermediate image transfer belt or the simple conveying belt.
The degradation of image quality ascribable to the speed difference between the drum and the belt at the first image transfer position obstructs further enhancement of image quality. Although fabrication technologies may be improved to reduce irregularity in drum radius or to increase eccentricity accuracy, such a scheme is undesirable from the cost reduction standpoint. While the drums, which are expensive, are replaced when they wear, this, of course, increases user's load.
[Problem 3]
To obviate so-called hollow characters or hollow pixels, Japanese Patent Laid-Open Publication Nos. 10-39648 and 62-35137, for example, propose to establish a certain speed difference between the drum and the belt or the sheet at the image transfer position. Assume that a speed difference or relative speed ΔVh (=Vd−Vb) is established at the first image transfer position; Vd and Vb respectively denote the moving speed of the belt or the sheet and the peripheral speed of the drum free from irregularity in radius. Further, assume that the angular velocity of the drum has a constant value of ωo while the drum radius is Ro, and that the length Ie of an exposed pixel for a unit period of time is Roωo. Then, the length I of the exposed image when the drum radius is Ro+ΔRo is expanded by ΔRoωo for the unit period of time, as expressed as:I=(Ro+ΔRo)ωo=Ie+ΔRoωo  Eq. (6)
The belt speed Vb is therefore Roωo−ΔVh, so that the speed difference ΔV of Roωo+ΔVh occurs at the first image transfer position. It follows that the pixel length varies by δI on the basis of the Eq. (3), as follows:                                                                         δ                ⁢                                                                   ⁢                I                            =                            ⁢                                                                    (                                                                  W                        1                                            +                      I                                        )                                    ·                  Δ                                ⁢                                                                   ⁢                                  V                  /                  Vd                                                                                                        =                            ⁢                                                {                                                            W                      1                                        +                    Ie                    +                                          Δ                      ⁢                                                                                           ⁢                      Ro                      ⁢                                                                                           ⁢                      ω                      ⁢                                                                                           ⁢                      o                                                        }                                ⁢                                                                   ⁢                                                      (                                                                  Δ                        ⁢                                                                                                   ⁢                        Ro                        ⁢                                                                                                   ⁢                        ω                        ⁢                                                                                                   ⁢                        o                                            +                                              Δ                        ⁢                                                                                                   ⁢                        Vh                                                              )                                    /                                      {                                          ω                      ⁢                                                                                           ⁢                                              o                        ⁡                                                  (                                                      Ro                            +                                                          Δ                              ⁢                                                                                                                           ⁢                              Ro                                                                                )                                                                                      }                                                                                                          Eq        .                                   ⁢                  (          7          )                    
Therefore, while the pixel is expanded by ΔRoωo for the unit period of time at the exposure stage, the pixel length is varied at the image transfer stage by δI:                                                                         δ                ⁢                                                                   ⁢                I                            =                            ⁢                                                {                                                            W                      1                                        +                                                                  (                                                  Ro                          +                                                      Δ                            ⁢                                                                                                                   ⁢                            Ro                                                                          )                                            ⁢                                                                                           ⁢                      ω                      ⁢                                                                                           ⁢                      o                                                        }                                ⁢                                                                   ⁢                                                      (                                                                  Δ                        ⁢                                                                                                   ⁢                        Ro                        ⁢                                                                                                   ⁢                        ω                        ⁢                                                                                                   ⁢                        o                                            +                                              Δ                        ⁢                                                                                                   ⁢                        Vh                                                              )                                    /                                      {                                          ω                      ⁢                                                                                           ⁢                                              o                        ⁡                                                  (                                                      Ro                            +                                                          Δ                              ⁢                                                                                                                           ⁢                              Ro                                                                                )                                                                                      }                                                                                                                          =                            ⁢                                                                    W                    1                                    ·                                                            (                                                                        Δ                          ⁢                                                                                                           ⁢                          Ro                          ⁢                                                                                                           ⁢                          ω                          ⁢                                                                                                           ⁢                          o                                                +                                                  Δ                          ⁢                                                                                                           ⁢                          Vh                                                                    )                                        /                                          {                                              ω                        ⁢                                                                                                   ⁢                                                  o                          ⁡                                                      (                                                          Ro                              +                                                              Δ                                ⁢                                                                                                                                   ⁢                                Ro                                                                                      )                                                                                              }                                                                      +                                ⁢                                  (                                                            Δ                      ⁢                                                                                           ⁢                      Ro                      ⁢                                                                                           ⁢                      ω                      ⁢                                                                                           ⁢                      o                                        +                                          Δ                      ⁢                                                                                           ⁢                      Vh                                                        )                                                                                        Eq        .                                   ⁢                  (          8          )                    
When the nip width W1 for image transfer is zero, the pixel is contracted by ΔRoωo+ΔVh. More specifically, the discussion that even when the drum radius is irregular, it does not vary pixels if the angular velocity of the drum is constant holds only if the nip width W1 at the first image transfer position is zero and if the speed difference ΔVh is zero. However, when the speed difference ΔVh is constant, the entire image is expanded (magnification error). This is also true when the drum has eccentricity.
It will now be seen that an error or contraction Ce occurs in the image due to the influence of the nip width W1 and sped difference ΔVh at the first image transfer position:Ce=W1·(ΔRoωo+ΔVh)/{ωo(Ro+ΔRo)}+ΔVh  Eq. (9)
Further, when the speed variation δV of the belt is added, i.e., when the speed difference ΔVh and the speed variation δV of the belt are established to obviate hollow characters, the following error E occurs:E=W1·{ΔRoωo+(ΔVh+δV)}/{ωo(Ro+ΔRo)}+(ΔVh+δV)  Eq. (10)
Japanese Patent Laid-Open Publication No. 2001-265081, for example, discloses an image forming apparatus configured to reduce the expansion or the contraction of a toner image despite the speed difference provided at the image transfer position for obviating hollow characters. This image forming apparatus uses a slip transfer type of image transfer system in which a speed difference is established between two surfaces facing each other at a first and a second image transfer positions. The speed differences at the two positions are opposite in sign to each other for thereby canceling the expansion or the contraction of a pixel, as will be described more specifically later.
Japanese Patent Laid-Open Publication No. 2000-338745 also shows a construction in which the peripheral speed of a drum and the moving speed of a sheet are equal, but the speed of an intermediate image transfer body is different. More specifically, a speed difference is established between the drum and the intermediate image transfer body so as to restore the original length of pixels at the second image transfer position.
We, however, found a case wherein the expansion or the contraction of a pixel could not be surely canceled due to factors not addressed to in the above two Laid-Open Publications.
[Problem 4]
We found an electrophotographic process in which the Eq. (1) held when the peripheral speed of the drum and that of the intermediate image transfer body differed from each other. More specifically, although the direction of the influence of the nip width W1 on the expansion or the contraction of a toner image was dependent on the sign of the speed difference ΔV, there was found an electrophotographic process in which pixels were thickened or expanded without regard to the speed difference ΔV, resulting in the deterioration of image quality. This will be described more specifically later.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 5-289455, 6-149084, 9-43932, 9-244422, 10-20579, 2001-34025, 2001-100614, 2001-265079, 2001-265081, 2001-318507, 2001-337561, 2001-343808 and 2002-174942 as well as in Japanese Patent Publication Nos. 7-31446 and 7-76850.